The present invention relates generally to the fabrication of semiconductor devices and, more particularly, to the reduction of gas flow from photoresist-coated wafers into the beamline during ion implantation.
Ion implanters are used in the fabrication of semiconductor devices to change the characteristics of a silicon or other semiconductor wafer by implanting a layer of dopant into the wafer. The dopant is implanted using ion implanters which generate and accelerate dopant ions and direct the energetic ions to a target wafer. By controlling the energy of the dopant ions, the depth of penetration of the ions into the silicon wafer can be controlled. Common dopants include boron, phosphorous and arsenic.
In ion implanters, a source module converts dopant gas molecules into a plasma. An ion beam is extracted from the source module. After extraction, the ion beam undergoes mass analysis and acceleration. Mass analysis is achieved by magnetically selecting specified ions in the beam so that they may be used for implant. Acceleration produces an ion energy commensurate with the desired implantation depth. Following analysis and acceleration, the ion beam is directed at the target wafer within a target chamber. The ion beam may be distributed over the wafer by beam scanning, by wafer movement or by a combination of beam scanning and wafer movement. Examples of ion implanter architectures are disclosed in U.S. Pat. No. 4,922,106 issued May 1, 1999 to Berrian et al., U.S. Pat. No. 4,899,059 issued Feb. 6, 1990 to Freytsis et al. and U.S. Pat. No. 5,350,926 issued Sep. 27, 1994 to White et al.
Many wafers processed by ion implanters use photoresist as an implant mask. When the ions impinge on the photoresist, large amounts of gas, primarily hydrogen gas, can be liberated, thereby increasing the pressure in the target chamber and the beamline. The increased pressure increases the probability that ions in the beam will collide with molecules of the liberated gas and suffer charge exchange or ionizing collisions. These collisions may change the charge state, direction and energy of the ions in the beam, resulting in a degradation of the ability to accurately measure the ion current delivered to the wafer and to control the location and depth of the ion implant. For example, neutralized ions are not measured by a Faraday beam current sensor, despite the fact that these neutralized ions are implanted into the wafer and contribute to total dose.
Efforts have been made to compensate for inaccuracies in ion beam measurements resulting from beam alteration attributed to introduction of extraneous species from various sources. Such efforts rely on sensing the residual background gas, and then adjusting the ion dose according to a calibration model based upon the probability of charge altering collisions occurring under the conditions of operation. Such techniques are described, for example, in U.S. Pat. No. 4,539,217 issued Sep. 3, 1985 to Farley, U.S. Pat. No. 5,319,212 issued Jun. 7, 1994 to Tokoro, U.S. Pat. No. 5,146,098 issued Sep. 8, 1992 to Stack and U.S. Pat. No. 5,814,823 issued Sep. 29, 1998 to Benveniste. Calibration, however, is difficult and not entirely satisfactory, as the calibration model may not accurately reflect the operating conditions. An ion dosage measurement apparatus for an ion implanter is disclosed in European Patent Application No. EP 0 964 426 A2, published Dec. 15, 1999. A restriction plate having an aperture is positioned in the beamline during a calibration procedure and is moved out of the beamline during ion implantation of wafers. An ion implanter incorporating a variable aperture for adjusting the amount of ion beam current passing therethrough is disclosed in U.S. Pat. No. 6,194,734 issued Feb. 27, 2001 to Loomis et al. None of the prior art approaches have been entirely satisfactory in resolving the problems which arise when gas is liberated from photoresist-coated wafers during ion implantation.
Accordingly, there is a need for improved methods and apparatus for limiting the adverse effects of unwanted gas on ion implantation.
The present invention is directed to an ion implanter apparatus which minimizes the effects on the ion beam of extraneous gases released into the target chamber as a result of beam impingement on target wafers. In particular, the invention involves dividing the target chamber into upstream and downstream regions with a divider having an aperture therethrough for passage of the beam to the target. The aperture size is adjusted to let substantially all of the beam through but to significantly limit gas flow from the target to the upstream side of the divider. Preferably, the aperture size is adjustable for different beam configurations and is located close to the target. In this manner, the inventive apparatus minimizes the beam volume which is exposed to extraneous species and confines collisions to a part of the implanter where they do not substantially affect ion direction, energy or charge state. The invention thus reduces the probability of beam-altering collisions and enhances the ability to control the dose and depth of the ion implant.
According to one aspect of the invention, a charged particle beam apparatus is provided. The charged particle beam apparatus comprises a charged particle beam source for directing a charged particle beam along a beam path in a downstream direction to a target, and a processing station that defines a target chamber. The processing station comprises a chamber divider which divides the target chamber into upstream and downstream regions during charged particle beam processing of the target. The target is located in the downstream region, and the divider has an aperture therethrough sized to permit passage of the ion beam to the target without substantial blockage and to limit backflow of gas into the upstream region of the chamber.
Preferably, the charged particle beam apparatus further comprises an aperture adjustment mechanism for adjusting the size of the aperture. The aperture adjustment mechanism may comprise one or more movable plates and a drive mechanism for moving the plates toward or away from each other to adjust the aperture size.
According to another aspect of the invention, an ion implanter is provided. The ion implanter comprises an ion source for directing an ion beam along a beam path toward a target, a mass analyzer disposed along the beam path for selecting desired ions in the ion beam, an accelerator disposed along the beam path for accelerating the selected ions in the ion beam to desired energies, a scanner for distributing the ion beam over the target, and a processing station that defines a target chamber. The processing chamber comprises a divider which divides the target chamber into upstream and downstream regions during ion implantation of the target. The target is located within the downstream region, and the chamber divider has an aperture therethrough sized to permit passage of the ion beam to the target without substantial blockage and to limit backflow of gas into the upstream region of the chamber.
According to a further aspect of the invention, a method is provided for reducing the probability of beam-altering collisions within a target chamber of an ion implanter. The target chamber is adapted for enclosing a target having photoresist thereon. The method comprises providing a divider within the target chamber which divides the target chamber into upstream and downstream regions during ion beam processing of the target. The target is located within the downstream region, and the chamber divider has an aperture therethrough sized to permit passage of the ion beam to the target without substantial blockage and to limit backflow of gas into the upstream region of the chamber.
According to a further aspect of the invention, an ion implanter is provided for implanting ions into a semiconductor wafer. The ion implanter comprises an ion beam generator for generating an ion beam, a processing station that defines a target chamber for receiving the ion beam, the processing station including a divider for dividing the target chamber into upstream and downstream regions during ion implantation of the semiconductor wafer, the semiconductor wafer being located in the downstream region, the divider having an aperture sized to pass the ion beam without substantial blockage and to limit backflow of gas from the downstream region to the upstream region, and first and second vacuum pumps coupled to the upstream and downstream regions, respectively, of the target chamber.